All industrial processes for the preparation of urea currently in use are based on direct synthesis according to the reaction:2NH3+CO2→CO(NH2)2+H2O  (1)Which takes place in two separate reaction steps:2NH3+CO2→(NH4)COONH2  (1a)(NH4)COONH2→CO(NH2)2+H20  (1b)
In the first step, there is an exothermic equilibrium reaction, kinetically favoured at room temperature, which however requires high pressures for reaching a favourable equilibrium at the high temperatures required in the subsequent step (1b).
In the second step, an endothermic reaction takes place, which reaches a significant rate only at high temperatures (>150° C.) with an equilibrium state which, at 185° C., leads to a conversion of only about 53% of the CO2, in a mixture of the reagents in a stoichiometric ratio. This unsatisfactory conversion can be conveniently raised by increasing the NH3/CO2 ratio, but it is further reduced in the presence of water. The latter also has an unfavourable effect on the overall kinetics of the process.
The above two reaction steps do not normally take place in separate areas of the reactor, but contemporaneously in the reaction mixture, which therefore comprises urea, water, ammonia, carbon dioxide and ammonium carbamate, with a relative concentration, in the different points of the reactor, depending on the different thermodynamic and kinetic factors which contribute to the process.
Processes for the production of urea by direct synthesis starting from ammonia and carbon dioxide have been widely indicated and described in the specific literature of the field. A large review of the most common processes for the production of urea can be found, for example, in the publication “Encyclopedia of Chemical Technology” Ed. Kirk-Othmer, Wiley Interscience, fourth ed. (1998), Supplement, pages 597-621.
In industrial processes for the production of urea, the synthesis is normally carried out in a reactor fed with NH3 and CO2 and aqueous solutions of ammonium carbamate coming from the recycled streams of the non-converted reagents, at temperatures ranging from 170 to 200° C., at pressures not lower than 13 MPa, with a molar ratio NH3/CO2 ranging from 2.5 to 4.5, calculated on the sum of the feeding streams, also including the reagents present as ammonium carbamate. The H2O/CO2 molar ratio fed to the reactor, generally ranges from 0.5 to 0.6. Under these conditions, the product discharged from the reactor has conversions ranging from 50 to 65% with respect to the total CO2 fed.
In addition to the water formed and excess of NH3 fed, the effluent from the reactor still has considerable amounts of CO2, mainly in the form of ammonium carbamate not converted to urea. The separation of urea from these products is effected, as is known, in various sections, operating at a high temperature and decreasing pressures, in which both the decomposition of the ammonium carbamate to NH3 and CO2 (products made available for recycling to the reactor) and the evaporation of the reaction water are effected, finally obtaining high-purity molten urea, sent to the final prilling or granulation step.
The separation and recycling section of the ammonium carbamate has investment and management costs which significantly affect the cost of the final product. From this section, all of the CO2 and part of the NH3, due to their contemporaneous presence, are made available for recycling as ammonium salts (carbonate and/or bicarbonate and/or carbamate, depending on the temperature and pressure) necessitating the use of water as solvent for their movement, in order to avoid the precipitation of the salts and the obstruction of the lines involved. This implies an increase in the amount of water present in the various liquid streams of the process and in the reactor, with the consequent negative effects on the conversion mentioned above.
Known processes which operate according to the above general scheme are described for example in U.S. Pat. No. 4,092,358, U.S. Pat. No. 4,208,347, U.S. Pat. No. 4,801,745 and U.S. Pat. No. 4,354,040.
In order to better clarify what is specified above, it should be pointed out that the amount of water recycled to the reactor for the above movement, is quantitatively in the order of that produced during the reaction. Traditional reactor is therefore particularly penalized as it is influenced, already in the feeding section of the reagents, by the high quantity of water coming from the recycled lines. Furthermore, the maximum water concentration is specifically in the terminal area of the reactor where, vice versa, it would be much more useful to have the lowest possible water concentration for favouring the shift of the equilibrium in step (1b) towards the right, specifically in this terminal area where the concentration of urea is already relatively high.
In order to overcome the above drawbacks and increase the conversion of CO2 to urea as much as possible in traditional plants, attempts have been made to operate at even higher temperatures and pressures even if this implies a further increase in the investment and running costs. Even in this case however the conversion levels do not exceed 60-65%.
In published European patent application nr. 727414 (in the name of the Applicant), a process is described in which at least a part of the recycled stream of ammonium carbamate coming from the medium and low pressure sections, is sent directly to the separation/decomposition step (called stripping, according to the commonly used English notation) included in the high-pressure synthesis section of the process, so as to be decomposed into ammonia and carbon dioxide, separated as gaseous stream, whereas most of the water remains in the liquid stream at the outlet of the stripper. This solution has effectively allowed the conversion to be increased per passage in the reactor, reducing the amount of water present, but to the detriment of the thermal balance of the stripping step, which requires an additional amount of heat to evaporate the liquid stream of the recycled carbamate. This can lead to an increase in the overall energy consumption and, in some cases, the necessity of modifying the equipment used for the stripping.
There is therefore still a considerable need for improvement in the production technology of urea suitable for making the synthesis process more efficient and economical.